ICs are typically manufactured many at a time in the form of dies on a semiconductor material wafer. After manufacturing, the semiconductor wafer is diced, so as to obtain a plurality of IC chips.
Before being packaged and shipped to the customers, and installed in various electronic systems, the ICs are tested for assessing their functionality, and in particular for ensuring that they are not defective. In particular, during the test, information regarding global or local physical faults (such as undesired presence of short circuits and break-down events) and more generally the operation of the IC on each die under test, may be detected (for example, by checking the waveform of one or more output signals generated by the IC on each die under test) so that only the dies that meet predetermined requirements proceed to the subsequent manufacturing phases (such as lead bonding, packaging and final testing).
According to a known testing technique, the IC dies are tested before the semiconductor wafer is diced into the individual chips. The test conducted at the wafer level is referred to as “wafer sort” or, as it is sometimes called, EWS (acronym for Electrical Wafer Sort)
For example, in the case of non-volatile semiconductor memory devices (such as Flash memories) the EWS is performed on each die wherein the memory device is formed, in order to verify the correct operation thereof.
For performing the test, a testing apparatus is used comprising a tester, which is coupled to the semiconductor wafer containing the dies to be tested, by means of a probe card, which is used for interfacing the semiconductor wafer to the tester.
The tester is adapted to manage signals that are employed for performing the test. Hereinafter, such signals will be referred as test signals and will be intended to include test stimuli (e.g., commands, addresses of memory locations, data to be written into the memory device) which are generated by the tester and are sent to each die to be tested by the probe card, and test response signals, which are generated by the ICs integrated in each die under test in response to the received test stimuli. The test response signals are sent by the IC integrated in each die under test to the tester, which processes them to derive an indication of the proper or improper operation of the ICs in the dies under test.
Probes are employed for the electrical coupling, through a physical contact, of the probe card with the dies to be tested, necessary for accomplishing the test signal exchange. For this purpose, the probe card consists of a PCB (Printed Circuit Board), which is connected to mechanical probes, which are adapted to physically contact input/output contact pads of each die to be tested.
In particular, each input/output contact pad consists of an enlarged metallization region surrounded and partially overlapped by a passivation layer.
During the testing, the metal contact pad is scrubbed by the mechanical scrubbing action of the probe tip. In such a way, the exchange of the test signals between the tester and the die to be tested may be accomplished.
Several probe card parameters contribute to improve the performance of the testing apparatus. Among the relevant probe card parameters, the planarity takes an important role. The planarity is the vertical distance (i.e., the distance along the Z-axis indicated in FIG. 1) between the highest and lowest probe tips of the probe card. The planarity affects the alignment between the probe tip and the contact pad so that, for example, a poor planarity may cause the scrub mark length and depth to be different from the desired scrub mark length and depth; this may modify the position in the plane defined by the X-axis and Y-axis in FIG. 1 as time passes. Another important parameter is the probe alignment accuracy, that is the position of the center of the scrub mark in the plane defined by the X-axis and Y-axis with respect to the center of the pad. In detail, scrub marks that are too long, or probe tips contacting the pad near the pad edge, may cause the passivation layer to be broken, while excessively deep scrub marks may damage the pad metallization region to the extent that the pads are then unusable for the testing and also for the normal operation of the IC. Damages to the input/output contact pads may also cause problems in the packaging phase of the die, since the input/output contact pads which are used for the testing are also used for bonding the die to the package.
The probe card parameters vary depending on the type of the probe cards.
Cantilever probe cards usually comprise a ring (for example made of aluminium, special alloys, or ceramic material) to which an epoxy holder is attached. Such epoxy holder is adapted to hold a plurality of testing elements in the form of resilient probes, made of an alloy having good electrical and mechanical properties. In particular, each cantilever probe includes a beam which is attached at only one of its ends to the epoxy holder, and at the other end the beam includes a tip which in use is intended to abut against a contact pad of the die with the IC to be tested.
A drawback of these probe cards is that some of their probe card parameters are unsatisfactory, so that, for example, alignment errors (such as an X-axis error, and/or an Y-axis error and/or a rotational error) between the probes' tips and the contact pads (that is, between wafer and probe card) may take place, with consequent potential problems of passivation layer breakage and with a negative impact on quality.
More specifically, the X-axis error and the Y-axis error represent the misalignment of the probes along the X and the Y directions, respectively, with the contact pads to be contacted. Due to the X-axis and Y-axis errors, the scrub mark on the contact pad may be off-center along the X direction and/or the Y direction.
In particular, during the testing of each die, rows of contact pads placed on opposite sides of the die are often contacted by the probes tips. In case of X-axis and Y-axis errors, all the probes may be misaligned in the same direction (X or Y), causing a displacement of the scrub marks along the X or the Y direction.
When the scrub marks of the contact pads on one side of the die are displaced towards the outer die edge, whereas the scrub marks of the contact pads on the other side of the die are displaced towards the inside of the die, a rotational error exists.
Such errors are emphasized by the fact that the cantilever probes are typically made based on mechanical techniques, so that the probes are different from each other. Such difference may further impair the values of the probe-card mechanical parameters.
In addition, this type of probe card may have some other limitations; for example, they may have a reduced parallel testing capacity: indeed, when several dies have to be tested at the same time, the number of mechanical probes significantly increases, and it may happen that the electrical contacts between the contact pads and the probes are not good and electrical discontinuities may take place.
Moreover, when the contact pads are located close to each other, it may be very difficult to ensure a good physical contact of the mechanical probes with the pads. Such problem may be emphasized when the pads are small in size and/or a large number thereof is present on each die.
In addition, even if only one probe is damaged, it may be necessary to replace the entire epoxy holder, and this negatively contributes to the increase of the overall cost of the test apparatus, and eventually of the ICs.
As an alternative to the cantilever probe cards, other known probe cards include a plurality of vertical (or pseudo-vertical) probes consisting of conductive wires passing through respective holes formed in a probe card head. In detail, the probe card head includes a top guide plate stacked on a bottom guide plate, which are suitably separated by an air gap. Each probe has a tip, which protrudes from the bottom guide plate and is adapted to electrically contact the corresponding contact pad of the die to be tested. A contact interface known as a “space transformer” (that is one of the possible contact interface types) is connected to the top guide plate and is adapted for electrically coupling the probes to the PCB (by means of corresponding wires) so that the exchange of the testing signal between the tester and the die under test may be accomplished.
A drawback of these probe cards is that they may have a poor scrubbing action on the contact pads, which may cause a reduced electric yield. In other words, the probes may not be able to adequately scrub the contact pad surface, that could have a metal oxide layer (and sometimes diffusion residuals) over the contact pad metallization region; the resulting probe scrub mark may not be able to ensure a good electrical contact between probe and pad, and if this happens the testing of the IC functionalities may be impaired.
Moreover, also in this case, when the contact pads are close to each other, it may be difficult to ensure a good physical contact of the probes with the pads. Such problem is emphasized when the pads are small in size and/or a large number thereof is present on each die.
A further drawback is that, similarly to the cantilever probe cards, when a probe breaks, it may be necessary to replace the whole probe head, thereby increasing the overall cost of the test apparatus.
Moreover, also in this case the probes are typically made based on mechanical techniques, so that the probes are different from each other. Such difference may further impair the probe card planarity.
Probe cards are also known which include MicroElectroMechanical Systems (MEMS) probes.
The probe cards having MEMS probes include of a multilayer ceramic head to which the MEMS probes are coupled. Each MEMS probe includes a base with contact features (such as a tip) adapted to contact the pad, and a beam, which is attached at on only one end thereof to the multilayer ceramic head and has the other end connected to the base.
Each MEMS probe is obtained by means of lithographic techniques, in particular a trench into a sacrificial substrate is formed (for example, by means of etching processes) and then filled with conductive material (e.g., gold) so as to obtain a base of the MEMS probe adapted for contacting the contact pad. The conductive material is then deposited on a portion of the sacrificial substrate in order to obtain the beam of the MEMS probe. Then, the MEMS probes are bonded to the multilayer ceramic head by means of wire bonds or other techniques and the sacrificial substrate is eliminated (for example, by an etching process). Finally, the multilayer ceramic head is connected to the PCB.
A drawback of MEMS probe cards is that even if only one MEMS probe is damaged, it may be necessary to replace the whole multilayer ceramic head, and this may be very expensive.
Other, different types of cantilever, vertical and MEMS probe cards are also known.